This invention relates generally to a method and apparatus for producing a pH change in a solution. More specifically, the invention relates to producing a pH change in a solution by irradiating the solution with visible light. With greater specificity, but without limitation thereto, the invention relates to using light to alter the pH of a solution to thereby cause an expansion and/or contraction of a pH dependent polymer immersed in the solution.
There exist a number of natural and synthetic fibers and gels that are expandable and contractible in volume when activated by an environmental change, such as exposure to a change min solvent composition, temperature, pH, electric field or photo irradiation, for example. As a commercially exploitable technology, the fibers and gels have applications in many fields, such as, for example, use in sensors, switches, motors, pumps, non-metallic operations and use in the medical and robotic fields where it is envisioned that these materials will be able to carry out the function of human muscle tissue.
The work of W. Kuhn and B. Hargitay as described in "Muskelahnliche Arbeitsleistung Kunstlicher Hochpolymerer Stoffe", Z. Elektrochemie 1951, 55(6), 490-502, incorporated by reference herein is one example of a synthesized polymer material capable of expansion and/or contraction. When the Kuhn and Hargitay polyacrylamide fiber, known as polyacrylic acid-polyvinyl alcohol (PAA-PVA), is placed within a solution of appropriately increasing pH, a 10% increase in fiber length is claimed to be observed.
Similarly, the work of T. Tanaka, D. Fillmore, S-T. Sun, I. Nishio, G. Swislow, and A. Shah described in the article "Phase Transitions in Ionic Gels" Phys. Rev. Lett. 1980, 45(20), 1636-1639, incorporated by reference herein discloses an observed 400% volume collapse for a polyacrylamide gel disposed in a 50% acetone-water solvent mixture in which the pH of the solvent is lowered at constant temperature and solvent composition.
The work of Kuhn and Hargitay as well as Tanaka and Fillmore et al use a typical approach to changing the pH of a solution. In this approach, the pH is changed by manually dripping an acid or base into the solution. This technique, known as the "acid drip" method, relies upon the rate of the diffusion of hydrogen ions to a polymer site and is considered undesirably slow for certain polymer applications, such as use in synthetic muscles.
Besides the pH activation method of Kuhn and Hargitay and Tanaka and Fillmore et al, there exist electrical polymer activation schemes in which p-electron conjugated conducting polymers and electronically doped non-conducting polymers are electrically activated (expanded and contracted). An example of this activation method has been characterized by Shahinpoor et al as described in the article of D. J. Segalman, W. R. Witkowski, D. B. Adolf, and M. Shahinpoor titled: "Theory and Application of Electrically Controlled Polymeric Gels", Smart Materials and Structures, Vol. 1 (no. 1), M. V. Gandhi and B. S. Thompson (Eds.), London: Chapman and Hall, 1992, 95-100. Like the pH activation method described above, the Shahinpoor et al method depends on the slow diffusion of ions to the active site of a polymer and therefore is also considered too slow for certain polymer applications such as use in synthetic muscles.
In addition to the activation approaches described above, there exist optical activation methods for causing volume changes in polymer fibers and gels. Noteworthy of these is the work of M. Irie and D. Kunwatchakun described in "Photoresponsive Polymers. 8. Reversible Photostimulated Dilation of Polyacrylamide Gels Having Triphenylmethane Leuco Derivatives", Macromolecules 1986, 19(10), 2476-2480. The Irie-Kunwatchakun studies were among the earliest on photoinduced volume changes in polymer gels. Photosensitive molecules, such as leucocyanide and leucohydroxide, were incorporated directly into a polymer's network. Irradiation with UV light produced a 2.2-fold reversible dimension change, but no significant volume change (phase transition) took place in the polymer studied, as the UV light-induced pH change was far from the pH null point of the polymer gel. Thus the magnitude of the dimension change was not optimized for certain applications such as robotics.
In the work of researchers Mamada and Tanaka as described in A. Mamada, T. Tanaka, D. Kungwatchakun, and M. Irie in "Photoinduced Phase Transition of Gels", Macromolecules 1990, 23, 1517-1519 and as described in A. Mamada, T. Tanaka, D. Kungwatchakun, and M. Irie in U.S. Pat. No. 5,242,491 titled: "Photo-Induced Reversible, Discontinuous Volume Changes in Gels" and issued Sep. 7, 1993, photoinduced phase transitions in gels were observed. The copolymer used was that of Irie-Kunwatchakun described above. At a given temperature, the polymer gel discontinuously swelled in response to UV irradiation and shrank when the UV light was removed. It is hypothesized that this swelling is due to dissociation into ion pairs, thereby increasing internal osmotic pressure within the gel. The shrinking process of this method is governed by ion diffusion and recombination, making the speed of the reverse process impossible to control, thereby hindering its usefulness in many polymer actuator applications.
In either of the UV studies described above, the UV radiation can cause undesired ionization, photolysis and molecular ligation of a utilized polymer.
Finally, in the work of A. Suzuki and T. Tanaka described in the article "Phase Transition in Polymer Gels Induced by Visible Light", Lett. Nature 1990, 346, 345-347, visible light was used to irradiate a gel containing a light-sensitive chromophore located in the backbone of an expandable and contractible copolymer. The chromophore absorbed the light and the light energy was then dissipated locally as heat by radiationless transitions, the result of which increased the "local" temperature of the polymer. Unlike the UV studies, the polymer expansion is a rapid process and is due to the direct heating of the polymer network by light. Yet the process of returning the polymer to its original size requires cooling, which becomes increasingly difficult as the temperature of the surrounding solution approaches the temperature of the polymer. This reverse process is too slow for many polymer uses such as in synthetic muscles.
Because many reactions are based on either acid or base catalyzations, including those of the polymers described above, researchers have investigated various approaches to promoting rapid pH changes. Such has been the case of Anthony Campillo et al as described in the article by A. J. Campillo, J. H. Clark, R. C. Hyer, S. L. Shapiro, K. R. Winn, and P. K. Woodbridge titled: "The Laser pH Jump", Proc. Intl. Conf. Lasers '78, Orlando, Fla., Dec. 11-15, 1978, Chem. Phys. Lett. 1979, 67(2), 218-222; the article by A. J. Campillo, J. H. Clark, S. L. Shapiro, K. R. Winn, and P. K. Woodbridge, titled: "Excited-State Protonation Kinetics of Coumarin 102", Chem. Phys. Lett. 1979, 67(2), 218-222; the article by J. H. Clark, S. L. Shapiro, A. J. Campillo, K. R. Winn, titled: "Picosecond Studies of Excited-State Protonation and Deprotonation Kinetics. The Laser pH Jump", J. Am. Chem. Soc. 1979, 101(3), 746-748; and U.S. Pat. No. 4,287,035 issued to John H. Clark, Anthony J. Campillo, Stanley L. Shapiro, and Kenneth R. Winn on Sep. 1, 1981.
The work of Campillo et al relies on excited-state proton transfer reactions to change the [H.sup.+ ] of a solution by several orders of magnitude. Campillo et al used a picosecond spectroscopy tool to directly measure excited-state deprotonation-protonation reaction rate constants. To promote a pH change, a UV laser with a pulse width of 20 picoseconds was used to excite 2-naphthol-6-sulfonate to a higher (S.sub.1) electronic state. From the measured rate constants, Campillo et al determined that the excited-state PK.sub.a value was 1.9, as opposed to the ground-state value of 9.1. This 7-unit change in pK.sub.a corresponds to a 7-order of magnitude increase in the acid dissociation constant, K.sub.a. Campillo's findings are consistent with earlier studies which show that excited-state K.sub.a values can differ from ground-state values by many orders of magnitude, see the disclosure of J. F. Ireland and P. A. H. Wyatt titled:"Acid-Base Properties of Electronically Excited States of Organic Molecules", Adv. Phys. Org. Chem. 1976, 12, 131-221.
Campillo et al claim that a major use of their technique is initiation of acid-base catalyzed ground-state reactions. For example, the reactants A and B are present in solution at pH 7. The ground state reaction, A+B.fwdarw.C, occurs only at pH 4. By exciting the Campillo et al "jump molecule", 2-naphthol-6-sulfonate, a subnanosecond jump from pH 7 to pH 4 can be achieved, thereby enabling the desired ground-state reaction. Referring to FIG. 1, a schematic state energy level diagram illustrates the path by which the "jump molecule" 2-naphthol-6-sulfonate travels to produce the pH change described. The 2-naphthol-6-sulfonate is irradiated with UV light and is excited from ground state S.sub.0 to first excited singlet state S.sub.1. Radiative decay (florescence) then occurs bringing the molecule back to its ground state.
A major drawback of the Campillo technique is the extremely short duration of the accompanying pH change, typically 10 nanoseconds. While Campillo proposes that the excited state duration, and hence pH change, could be prolonged through use of repetitious irradiation, such an irradiation would require a bombardment of photons on the order of a million times a second. An additional shortcoming of the Campillo technique, when utilized with expandable and contractible polymers such as those described above, is that the utilized UV radiation promotes undesirable polymer ionization, photolysis and other molecular ligation. Additionally, the extremely narrow illumination path (0.1 mm or 5D-6 cubic centimeters) provided by the utilized 266 nanometer laser is considered insufficient to effectively illuminate an expandable/contractible polymer to undergo an appreciable change in volume.